1. Field of the Invention
The present invention relates to gas mixtures used in high pressure plasma processes and, in particular, to high pressure plasma processes that use a nonflammable ternary gas mixture.
2. Background Art
High pressure plasmas are used for various plasma arc machining processes, such as metal cutting, metal gouging and metal coating deposition. Unlike low pressure systems, high pressure plasma systems can be operated both at atmospheric pressure and reduced pressures. A low pressure plasma can only be operated under reduced pressure environments. The part of the plasma apparatus used in these high pressure plasma processes from which the plasma issues is frequently referred to as a plasma torch or gun. The generation of a high pressure plasma requires the presence of a gas, typically introduced into the nozzle at high velocities. In a plasma, gas molecules are partially ionized into charged particlesxe2x80x94ions and electrons. Accordingly, because of this ionic nature, plasmas conduct electricity. A hot zone is formed in the plasma when the ions and electrons recombine to give off intense heat and light. Plasma arcs operate typically at temperatures of 10,0000-14,0000xc2x0 C.
Plasma arc cutting refers to thermal cutting processes that sever or remove metal by melting it with the heat of an arc between an electrode and the workpiece. Plasma gouging is a thermal cutting process variation that removes metal by melting or burning a portion of metal which is completely removed to form a bevel or groove. In both plasma arc cutting and gouging, electricity is conducted from the torch to a metallic workpiece. This is accomplished through an electrically conductive electrode within the plasma torch. Furthermore, in both these processes, the plasma used to melt the metallic workpiece is constricted and confined to a small area. Constriction is accomplished by a nozzle (often made from copper) which constricts the high velocity gas causing a tornado effect. This tornado effect provides energy to the arc that melts and blows away the metal.
A particularly useful high pressure plasma coating process is the Plasma Transferred Wire Arc (xe2x80x9cPTWAxe2x80x9d) process. The PTWA system produces high quality metallic coatings for a variety of applications such as the coating of engine cylinder bores. In the PTWA process, a high pressure plasma is generated in a small region of space at the exit of the torch. A continuously fed wire impinges upon this region where the wire is contiguously melted by the plasma. The plasma torch takes the molten particles and directs them towards the surface to be coated. PTWA systems are high pressure plasma systems. High pressure plasmas are also utilized in metal cutting or gouging torches. The PTWA torch behaves much like a plasma cutting or plasma gouging torch, but produces a coating unlike the metal removal torches. Instead of allowing the molten droplets to be removed as is the case for metal removal torches, the droplets are directed onto a prepared surface in a defined pattern. The metal removal torches also operate at somewhat higher voltages than coating deposition torches.
Constant current direct current (xe2x80x9cDCxe2x80x9d) electrical power supplies are used for the operation of most plasma torches. These power supplies maintain a user defined current level while the voltage is determined by the properties of the gas mix used and the geometry of the torch. The voltage multiplied by the current gives the total of amount of power that the torch is generating. The voltage of a plasma torch indicates the difficulty with which the molecules are ionized. Higher voltages indicate more difficulty in the ionization of the gas molecules. Therefore, a gas mixture which has a characteristically high voltage gives off greater amounts of heat energy during the process of recombination. By design, higher voltages are needed for the operation of the PTWA system than other coating deposition plasma torches. Few gas mixes can generate this kind of voltage without being flammable. Gas mixes containing oxygen can be used for this purpose. However, there are issues associated with using oxygen. The use of oxygen gas mixes require the use of a different cathode material such as zirconium or hafnium. Thoriated tungsten cathodes are preferred because of their extended lifetime. However, they are very sensitive to the presence of oxygen.
Typically, the PTWA method uses a gas mixture of 65-75% argon and 25-35% hydrogen. This gas mix provides sufficient energy needed to melt a continuously fed wire. The molten particles are subsequently used to form a metallic coating on a specified part. Unfortunately, this gas mixture is flammable, and requires the installation and use of fire protection and safety systems. The U.S. Department of Transportation sets a flammability threshold of 6.2% hydrogen by volume in an inert gas. Anything higher than this threshold is considered flammable.
U.S. Pat. Nos. 6,126,858 and 6,265,687 disclose ternary gas mixtures of helium, argon, and hydrogen useful for use in a low pressure plasma projection process. Both of these patents disclose ternary gas mixtures that include hydrogen in an amount of 5.5% to 15% by volume.
Concentration of hydrogen above 6.2% are classified as flammable by the U.S. Department of Transportation, while concentrations below this amount classified as nonflammable. Flammable hydrogen gas mixtures pose significant problems relating to storage and shipping of such mixture. U.S. Pat. No. 4,937,500 (the ""500 patent) also discloses a ternary gas mix useful for closing switches in diffuse glow discharge. The ""500 patent does not disclose a high pressure plasma process nor does it disclose ternary gas mixtures of hydrogen gas.
The present invention overcomes the problems encountered in the prior art by providing in one embodiment a nonflammable ternary gas mixture to be used in a plasma machining operation. The ternary gas mixture of the present invention comprises, more preferably consist essentially of, and most preferably consists of about 4% to about 6.2% hydrogen by volume, about 30% to about 50% argon by volume, and the balance nitrogen. The reduced amount of hydrogen classifies the ternary gas mixture of the present invention as nonflammable under DOT standards.
In another embodiment of the present invention, a method of generating a high pressure plasma is provided. The method of this embodiment comprises introducing the plasmagenic gas mixture of the present invention into a plasma torch apparatus, initiating a plasma in the plasma torch apparatus, and sustaining the plasma with a high voltage between a cathode and an anode.